In general, it is more common for citizens to be required to serve in the military of their own country. This is because the military is seen as a way to defend the country and its citizens from threats from other countries. However, there are some countries that allow citizens to choose whether or not to serve in the military. This is often the case in countries that have a strong tradition of individual liberty.
There are also some countries that allow citizens to serve in the military of a different country. This is often the case in countries that have close alliances with each other. For example, many citizens of the United States serve in the military of Canada, and vice versa.
If you are interested in serving in the military, it is important to research the laws and policies of the country you are interested in serving in. This will help you to understand your rights and obligations as a citizen.